Physical aging of ester-cross-linked hollow fiber membranes for natural gas separations and mitigation thereof

Like other glassy polymers, ester-cross-linked polyimide hollow fiber membranes can experience physical aging, which leads to some loss of permeation productivity. Highly quenched asymmetric structures are especially susceptible to such aging. To explore approaches to control physical aging, CO2 conditioning and continuous CO2/CH4 feed tests were studied in this work. A 100 psig CO2 conditioning overnight did not affect the aging of cross-linked hollow fibers significantly. However, periodical 15 psig CO2 conditioning reduced the CO2 permeance loss of cross-linked hollow fiber membranes by ~50%, compared to unconditioned ones during a 2300 h aging test. To further suppress physical aging, continuous exposure to CO2/CH4 feed typical of realistic continuous operations was used for hollow fiber samples. This protocol, in fact, reflects the realistic situation for use of asymmetric membranes that typically run under protracted steady state conditions. A 200 psia continuous 50/50 CO2/CH4 feed resulted in a CO2 permeance loss of only up to 3%, compared the 25% loss for samples without continuous mixed gas feed during a 400 h aging test. These results suggest that physical aging may be essentially quenched as long as the membrane remains contacted with a high CO2 partial pressure feed typical of actual aggressive feeds for which the membrane would be used. By inhibiting physical aging, periodic CO2 conditioning and continuous CO2/CH4 feed provide high performance of cross-linked hollow fiber membranes for natural gas separations.